1. Field of the Invention
This invention relates to a photosensitive resin composition. More particularly, it relates to a photosensitive resin composition for forming a protective coating film having excellent characteristics which can be used either alone, to provide a photosensitive, tacky coat; or it can be combined with a variety of other types of ingredients such as a crosslinker to increase crosslinking density, a resin to remove tackiness, or even another photosensitive ingredient to increase sensitivity to light. The uses of the instant photosensitive coating material thus includes photoresist applications such as plating resist, etch resist and solder mask.
Soldering masks are used to produce printed wiring boards. The solder masks function is basically to prevent soldering bridges, to keep electric insulation between conductors thus preventing conduction between solder areas and to prevent the corrosion of a naked copper conductor. It is also desirable to have photoimagable solder masks which are able to retain an image which can be used as a blueprint on which the solder is placed. Due to the desirability of increasing wiring density, it is desirable to use a solder mask having precise resolution and extremely good electrical insulation properties. The instant photosensitive composition can provide these advantages.
Known solder masks are described in U.S. Pat. No. 4,499,163 which describes a photosensitive resin composition containing urethane diacrylate or dimethacrylate, a linear polymeric compound having a glass transition temperature between about 40.degree. and 150.degree. C., and a sensitizer which generates free radicals in actinic light. The linear polymeric compound described includes the vinyl series linear polymers or copolymers such as, for example, polymers made from vinyl monomers including methyl methacrylate, butyl methacrylate, methyl styrene, vinyl toluene, and the like. The sensitizer for generating free radicals includes, for example, substituted and unsubstituted polynuclear quinones.
Another photocurable urethane acrylate resin composition which also is described as being usable for permanent resists, can be found described in U.S. Pat. No. 4,587,201. The composition therein described uses a urethane-acrylate resin which is composed of polybutadiene polymer. This resin is combined with a photo-polymerization initiator to form the permanent resist material.
Most photoimagable solder masks lack the ability of being developed in an aqueous solution. This necessitates the use of organic solvent developing solutions. The use of such organic solvents has become undesirable due to the environmental regulations for solvent emissions control.
Another disadvantage found with solder masks that use organic solvent developing solutions, is the continued sensitivity of the final product to organic solvents such as methylene chloride. In certain product applications, moreover, such susceptibility is undesirable.
It is therefore desirable to develop photoimagable solder masks coating compositions which can be developed in aqueous solutions, and which are resistent to organic solvents such as methylene chloride. It is an object of the instant invention to provide such a coating composition. It is also an object of the instant invention to provide a solder mask coating composition which can be stripped with commercial alkyline strippers. Accordingly, the photosensitive resin compositions described herein have excellent resolution, flexibility, adherance to metals, solvent resistance, high temperature resistance and electrical insulation properties.
Although the instant compositions are highly viscous photosensitive materials which can be useful as additives for compositions that require both thickening and the addition of a photosensitive coating material, the function to which the instant compositions are substantially directed is the photoimageable solder mask.
In addition to aqueous development and solvent resistance, the photocureable coating polymers described herein have excellent adherence to the metal substrates. When used as a solder mask, the coating is exposed to the extreme heat of the liquid solder, and yet the instant coating polymers still have good adherence.
Embodiments are described herein; however, that combine the instant solder mask coating compositions with materials that give it even better metal adherence and provide a liquid material having viscosity and flow properties that adapt the coating composition to different types of solder mask application machinery.